As the process of arc welding is perhaps one of the most widely used manufacturing processes in the world, great efforts have been made in the last few years to automate the process by using optically controlled robotic welders. This is especially advantageous from the standpoint of increased productivity and the ability to make uniform, high-quality welds.
In order to automatically control the welding process, it is necessary to measure and determine certain parameters of the weld pool as well as determining where the weld seam is so as to guide the welding torch thereover. In the past, only the welder has been able to view the arc length and the weld pool contour and to adjust the torch accordingly to achieve optimum penetration of the weld. Additionally, the welder is in a position to readily track the sometimes irregular weld preparations or joints.
Early attempts have been made to automatically track weld preparation areas by use of sensors which look ahead of the welding torch. Such sensors attempt to discern where the weld preparation area or joint is and adjust the movement of the welding torch so as to coincide therewith. Such methods have utilized both direct contacting type sensors, which are dragged through the weld preparation area ahead of the welding torch, and by indirect measurement techniques, such as the use of infrared detectors.
In recent years, video equipment has been used to attempt to monitor the welding operation. These methods entail positioning a television camera to obliquely view a welding operation, which yields a view similar to what a welder would see. Such a view of the welding process has the inherent problem of viewing a very bright welding arc, which tends to "wash out" the view unless appropriate filters are utilized. These filters limit the view of the camera to one or several wavelengths of light rather than the broad spectrum available from the arc. Problems of obliquely viewing the welding operation with a video camera are that unforeseen obstructions to the camera in real welding situations arise. Such obstructions can be caused by the welding preparation geometry and by constraints on the placement of the camera. Also, parallax distortions of the image of the weld puddle are caused by the oblique viewing position. These problems are in addition to the over-exposure problems caused by direct arc viewing as described above.
As is known, a relationship exists between the actual contour of the molten pool and the penetration being achieved by the welding process. Thus, it is desirable to be able to directly measure and control the weld pool contour. Precise control of the weld puddle contour produces a correspondingly precise control of penetration being achieved by the welding process. Prior methods of analysis of video data from the arc and weld pool area have made the assumption that the bright areas represent reflections from the weld pool, and that once the light intensity has decreased to a certain value, then the edge of the weld pool has been approached. Such methods employ a binary go/no go logic system to establish the weld pool width. Unfortunately, oscillations of the size and contour of the weld pool exist due to fluctuations in arc voltage, addition of filler wire, and forward motion of the electrode along the weld preparation area. Such oscillations cause the area of brightness to vary considerably. Attempts to mitigate this error are simply corrections of the data rather than actual measurement and evaluation of the true weld pool contour.
Perhaps the biggest problem involved with providing reliable control of the welding process is seam tracking. Sensors which measure the precise location of the welding seam have been used with limited success. Unfortunately, in order to avoid damage to the sensor and also to keep the sensor from being obscured by the arc area and the molten metal, it has been necessary to sense the seam some distance ahead of the welding torch. This immediately produces the requirement of having some delay in system response so that the system response to seam tracking occurs at the time when the welding head is over the area of change.
Seam tracking devices have been of two types. First are those that use the arc itself as the sensor. Sensors of this type sense voltage and current variations of the arc when the various surface features of the base metal are encountered. One such method oscillates the arc back and forth across the seam while measuring voltage changes as the arc gets longer as it approaches the weld preparation. Various schemes have been proposed to allow for this oscillation. Both magnetic and mechanical motion devices have been utilized previously. The second method is the direct arc viewing method for seam tracking at the point of welding. Here, the methods identify the edges of the weld preparation or weld groove from analyses of light from the arc reflected off the edges from the grooves or side walls of the weld preparation area. A feedback system is then provided to respond to the reflected light to produce the desired effect of following the seam to be welded. As mentioned, the problems of tracking the weld seam or weld preparation area by viewing the area ahead of the weld require that any information thus received be delayed before it is implemented so that the welding torch is indeed over the area detected, or the change detected, at the time the change instruction or signal is initiated. Also, any process control data, whether it be used for seam tracking or for viewing welding parameters in the area of the arc in the molten pool, are subject to error due to parallax from the oblique view of the camera positions known in prior art and the undesirable masking of the far side of the weld pool by the arc itself.
Attempts to overcome the problem of positioning a camera to obliquely view a welding operation have resulted in an apparatus disclosed in U.S. Pat. No. 4,532,408 (Richardson). This patent discloses a welding torch having a through the torch viewing system which provides a view of the welding operation which is coaxial with the welding electrode. This allows the electrode to block the intensely bright light from the welding arc. This view may be coupled to a video picture analyzer, which may then be used to control a robotic welder. Alternately, this view may be coupled to a monitor, which in turn is observed by an operator who can control the welding operation from a remote location.
Problems with this method are that only one view is provided to control the welding operation. In order to adequately control a welding operation by use of optically controlled robotics, it is believed that at least two views of the welding operation are needed. One view would be coupled to a seam tracking analyzer to track the welding electrode along the weld preparation area, while the second view would be utilized by weld pool monitoring equipment, which monitors parameters of the weld pool and determines penetration of the weld.
Accordingly, it is an object of this invention to provide a welding torch equipped with a beam splitter which provides a reflected view and a transmitted view of the welding operation. These views are coaxial with the welding electrode, and in conjunction with a cooled electrode holder which consists of a single spoke or arm, provides views of the welding operation which are relatively unobstructed. Further, the beam splitter may be fitted with filters which block certain wavelengths of light. Still further, the beam splitter may be selected to reflect certain wavelengths while passing other wavelengths. This is a consideration when lasers are used to detect parameters of the weld pool.